When I saw that FamilySearch.org recently added more indices to its California County Birth and Death Records, 1849-1894 collection, I searched again for Dennis and came up with a hit.
|Death Certificate for Den Creedon, 1929, French Camp, CA|
There are several new pieces of information on the death certificate, the most intriguing being that Dennis was a widower. The certificate states that he had lived in California for 10 years and that he was buried in the San Joaquin General Hospital Cemetery, #2524 on January 19, 1929. The informant was J. Hogan. If he was in California for 10 years, then he should be in the 1920 census, but I haven't had any luck there so far.
Posts on RootsWeb in the CASANJOA-L Archives state that the graves in the San Joaquin General Hospital Cemetery were moved to either the Cherokee Memorial Park in Lodi, CA or the Stockton Rural Cemetery in Stockton, CA in 1989. I didn't find anything on FindAGrave for any of these locations, so the search continues.